This type of electromagnetic clutch is suitable for a compressor of an automotive air-conditioning system and is capable of intermittently transmitting the driving force of an engine to the drive shaft of the compressor. A conventional electromagnetic clutch disclosed in Unexamined Japanese Utility Model Publication No. H07-35830, for example, has a rotor rotatably supported on one end portion of a compressor housing. A driving belt is passed around the rotor and a pulley of the engine, and the rotor has an electromagnetic coil therein. The coil is arranged inside a stator and serves as an electromagnetic force generator. An armature in an annular plate like shape, as a clutch plate, is coupled to the drive shaft of the compressor through a coupler in a manner such that the armature is allowed to be attracted to the rotor by the electromagnetic force produced by the electromagnetic force generator. More specifically, the coupler has a hub fitted around the drive shaft of the compressor, and part of the hub projects from the aforementioned end portion of the housing. A flange extends integrally from the projecting part of the hub, and the armature is supported by the flange through a plurality of leaf springs and located coaxially with the rotor. The leaf springs constantly urge the armature in a direction away from the rotor, and accordingly, when the electromagnetic coil is not excited, the armature is kept separated from the rotor with a predetermined gap therebetween. On the other hand, when the electromagnetic coil is excited, the armature is attracted to the rotor by the electromagnetic force of the electromagnetic force generator against the forces of the leaf springs and is pressed against the rotor with a predetermined surface contact pressure corresponding to the attractive force. If, at this time, the rotor is being rotated by the driving belt, the armature is dragged and rotated with the rotor, whereby the drive shaft of the compressor is rotated through the leaf springs, the flange, and the hub.
In the conventional electromagnetic clutch, the surface contact pressure acting between the armature and the rotor is given solely by the electromagnetic force produced by the electromagnetic force generator, that is, the electromagnetic coil, as stated above. Accordingly, in order to increase the torque transmitted to the drive shaft of the compressor, measures need to be taken to increase the size of the electromagnetic coil and of the rotor or to supply more electric power to the electromagnetic coil.
Increasing the torque by these measures, however, hinders reduction in weight of the electromagnetic clutch as well as in electric power consumption. Also, increase in size of the electromagnetic coil leads to increase of the self-inductance, and thus, when the supply of electric power to the electromagnetic coil is stopped, the armature cannot be readily separated from the rotor. As a result, a loud noise is produced when the armature separates from the rotor. On the other hand, when the power supply to the electromagnetic coil is started, the armature is synchronized with the rotating rotor in a short period of time because of the increased electromagnetic force. Accordingly, the torque of the rotor is transmitted to the armature as an impact force, with the result that high load instantaneously acts upon not only the parts constituting the clutch but the parts constituting the compressor.